There is a need in the art for comfortable, convenient and inexpensive means for prewarming air for breathing. For example, in the case of persons suffering from cardiac insufficiency, whether congenital or acquired as a result of heart damage resulting from injury or disease, the patient is commonly advised to engage in moderate exercise, as tolerated. One of the best of such exercises is walking, which is of course normally an outdoor activity. This, however, may lead to problems, particularly in cold weather. Under cold conditions, the heart is doubly stressed--partially by the physical demand on the muscles, requiring increased circulation to carry away lactic acid and other waste products or muscular activity, and partially by the additional increased circulation required to maintain the body temperature at a proper level. The latter stress does not signal its presence by muscular discomfort, as does the former, and the result is that the patient is unconsciously placing greater demands on his heart than he should. The same effect is commonly experienced by persons having no previous history of cardiac problems, as one is reminded every year by an upswing in incidence of heart attacks which can be directly associated with heavy exercise such as snow shovelling during periods of cold weather. One of the most direct mechanisms whereby the body is chilled is by inhalation of cold air which not only chills the tissues forming the walls of the components of the respiratory tract, but also chills the bloodstream itself by heat transfer which takes place in the alveole concurrently with the oxygenation of oxygen-depleted blood from the pulmonary artery. Thus, the demands placed on the heart for maintenance of body temperature can be alleviated and the above mentioned problems can be overcome to an appreciable degree, by preheating air before it is inspired into the body.
Previous attempts to solve this problem have generally taken the form of interposing a porous barrier, for example of knit wool or spongy polymer, between the nose and/or mouth of the user and the external atmosphere. While successful to a degree, such measures suffer from certain disadvantages. For example, the interposed porous material necessarily involves some resistance to free air flow, and to that extent makes breathing more difficult. Also, there is an annoying tendency for moisture in exhaled breath to condense on contact with the cold ambient atmosphere, and to deposit as moisture or as ice crystals on the surface and in the pores of the porous material, creating a most uncomfortable condition. Perhaps, more importantly, the mechanism whereby the warming of incoming air is accomplished is by direct heat exchange with warm exhaled air in the porous material. This necessitates that a certain amount of the exhaled air be trapped in the porous material, mixed with fresh incoming air to accomplish the desired heat exchange, and then reinhaled as part of the mixture. This, in turn, defeats pro tanto the advantage of the heat exchange, inasmuch as the mixture being breathed has already been partially depleted of oxygen, so that additional quantities of the mixed gases must be inhaled in order to maintain the required oxygen supply.
The following patents are illustrative of prior art efforts to solve these problems.
U.S. Pat. No. 909,979 granted Jan. 19, 1909 to E. E. Zerkle for "Respiratory Apparatus" fails to disclose any means for warming the fresh intake air.
U.S. Pat. No. 2,344,920 granted Mar. 21, 1944 to G. P. Maggi for "Frost Mask" shows heat exchanging tubes that pass through a warm chamber inside a mask in which the incoming air is subjected only to a small temperature gradient within one or more tubes of limited heat transfer surface.
U.S. Pat. No. 3,249,108 granted May 3, 1966 to L. A. Terman for "Mask for Protecting Respiratory Tract" shows an electrical or chemical heating element within a face mask to warm fresh, cold air that is drawn into the apparatus.
U.S. Pat. No. 3,491,754 granted Jan. 27, 1970 to W. W. Weese for "Methods and Apparatus for Facilitating Respiration" shows a multiple layered heat exchanger for warming fresh air prior to inhalation by the wearer through a mouth piece.
U.S. Pat. No. 3,707,966 granted Jan. 2, 1973 to J. A. Nebel for "Personal Breathing Apparatus" shows a chest pad for preheating breathing air that is supplied to a face mask through a flexible conduit.
U.S. Pat. No. 4,062,359 granted Dec. 13, 1977 to M. E. Geaghan for "Low Temperature Breathing Apparatus" relies on a porous tube that can be placed between the body and adjacent clothing to preheat air that is being inhaled.
U.S. Pat. No. 4,150,671 granted Apr. 24, 1979 to H. T. Tiger for "Warm Air Weathermask" uses exhaled breath as a source of warmth for the incoming cold air.
Further in this respect a clinical study by E. N. Schachter, M.D., Elliot Tack and Marjorie Lee, M.D. published by Home Health Care Products, Medical Products Division/3M, 555-1S, 3 M Center, St. Paul, Minn. 55101 in April 1979 and titled "3M Cold Weather Research Data & Results" describes a number of tests conducted on young asthmatics using, presumably, the 3M cold weather mask which is formed from soft foam material that is supposed to warm and moisten inhaled air.
All of these foregoing designs have a number of flaws or disadvantages, some of which have been considered at some length above. The porous nose mask, for example, produces a great deal of moisture condensation and rebreathing of exhaled air. Those devices, moreover, which couple the mouth or nose piece to a heat exchanger through flexible tubing are unsatisfactory for other reasons. Hand mobility is seriously impeded with these tube devices. The additional weight of the tube, stretched as it is between the heat exchanger and the mouth or nose piece places a further and uncomfortable strain on the neck muscles. The exposed span of tubing between the wearer's body and the mouth or nose piece provides a major locus for lost heat. The chest and back of the wearer also are not necessarily the best radiators of body heat, nor is the location of the heat exchanger, buried under layers of clothing, a suitable location for the air inlet to an heat exchanger because of the pressure loss and attendant fatigue in respiration that is experienced when inhaling air through this resistance.
Accordingly, there is a need for a device that warms cold air through body heat without introducing inordinate breathing resistance, heat loss or discomfort.